Nokia 301 Service Manual page 166

RM-839; RM-840; RM-841
System Module
Headset interface (external earpiece + external microphone)
The output driver circuit for headsets are often denoted as the "XEAR path" as it's a driver dedicated for
external audio like headset, headphone etc.
The output signal of Left channel is differential signal between HS_EAR_L and HS_GND, same situation on right
channel. The serial resistor(R2057,R2058) on both HS_EAR_L and HS_EAR_R trace is 33R.
Figure 58 Headset interface
External microphone bias circuit
The external microphone circuit is configured in pseudo-differential mode (Wheatstone bridge) for better
noise performance. The affected components in the Wheatstone bridge are: R2830, R2834, R2088 (this
component is in figure XX: headset interface).
External microphone is also useful for accessary detection. affected component are: R2832, R2833, R2089
(this component is in figure XX: headset interface).
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